Corrosion inhibition in chlorite bleaching of cellulose derivatives



United States Patent Bayard T. Lamhorn, Matawan, and Richard A. Leonard,

Metuchen, N. J., assignors to Hercules Powder Compauy, Wilmington, DeL, a corporation of Delaware No Drawing. Application May 17, 1951, Serial No. 226,936

5 Claims. ((11. 260--230) This invention relates to a noncorrosive bleaching composition. In a specific aspect this nvention relates to a process for the bleachingof cellulose derivatives with a chlorite bleaching agent and to the composition of matter employed in the bleaching step. In a more specific aspect this invention relates to a process for in hibiting the corrosive action of metal chlorites on metallic reaction chambers in a cellulose acetate bleaching process and to the noncorrosive bleaching composition employed in the bleaching step.

Cellulose derivatives, for example, cellulose ethers and esters, and particularly cellulose acetate, are employed in high temperature operations, such as molding, for the formation of a wide variety of articles, and in such high temperature operations it is essential that the cellulose derivative have a high thermal color stability in order that the product resulting from the process will be color less. It has recently been found that cellulose derivatives that have been bleached with a chlorite, for example, an alkali metal or alkaline earth metal chlorite, have a thermal color stability higher than that of cellulose derivatives which have been bleached with other known bleaching agents. In carrying out the bleaching process with the chlorite bleaching agent an aqueous slurry of the cellulose derivative is formed, and the pH of this slurry is adjusted with an acid to obtain a pH lower than 7. The cellulose derivative is then bleached by introduction of the chlorite bleaching agent. Various acids can be used for the pH adjustment, but acetic acid is regarded as preferable since it is a relatively weak acid, and it has no appreciable oxidative action upon the cellulose derivative. However, the chlorite, when employed in the presence of an aqueous slurry of a cellulose derivative whose pH has been adjusted'with acetic acid, has an undesirable corrosive action upon the metallic surfaces of the reaction chamber, even when the reaction chamber is constructed of a corrosion resistant stainless steel. Further, the bleaching of various pulps, such as paper pulp and wood pulp, is practiced extensively commercially, and among the bleaching agents employed is chlorine dioxide which is also corrosive upon.

metallic surfaces particularly when it is employed in an aqueous medium having a pH lower than 7. Obviously, such corrosive action is undesirable and the importance of a method for inhibiting such corrosion is evident.

It is an object of this invention to provide ;a novel process for the treatment of cellulosic derivatives.

It is another object of this invention to provide a novel process for the bleaching of cellulose esters with a chlorite bleaching agent.

It ,is another object of this invention to provide a novel bleaching composition containing either chlorine dioxide or a chlorite bleaching agent and a corrosion inhibitor.

It is another object of this invention to provide a novel process for inhibiting the corrosive action of metal chlorites on metallic reaction chambers in a cellulose acetate bleaching process.

It is a further object of this invention to provide a novel noncorrosive bleaching composition.

Further and additional objects of this invention will be apparent from the detailed disclosure.

In accordance with this invention it has been found that the corrosive action of an aqueous solution of .a

chlorite or chlorine dioxide bleaching agent upon metal ,Massachusetts, 1936, prepared under the ICC surfaces can be inhibited by adding to such an aqueoussolution a phosphoric acid or a derivative of said acid whose aqueous solution has an acidic pH. In bleaching a cellulose ester, for example, cellulose acetate, it is preferred to employ phosphoric acid to adjust the pH of the aqueous slurry of the cellulose ester in place of acetic acid or other acid. It has further been found that when phosphoric acid is used for pH adjustment in such a bleaching process the cellulose ester resulting from the bleaching operation has a lower and more desirable color than those cellulose esters that are bleached in the presence of acetic acid or other acid.

The following p cific examples d mons r t h utili nd d sira ili y of hi inv n on- EXAMPLE 1 Test pieces of several stainless steels were each immersed in separate aqueous solutions of sodium chlorite and the solutions were then heated for 5 hours at C. Subsequently, each piece of stainless steel was rubbed with a rubber stopper to remove any loose coating, and then each piece or steel was washed, dried, Weighed, and the amount of corrosion was calculated as inches of penetration per year. Solution A contained 0.7 gram of sodium chlorite per liter and the pH of the solution was adjusted to 4 with acetic acid. Solution contained 0.7 gram of sodium chlorite per liter and its pH Was .adlusted to 4 with phosphoric acid. The stainless steels employed were of the auStenitic chromium-uiclgel type. The stainless steel for run l contained a maximum of 0.08% carbon, 18-20% chromium, 55-11% nickel, and a maximum of 2% manganese. The stainless steel for run 2 contained a maximum of 0.10% carbon, 16-18% chromium, 10-14% nickel, and 23% molybdenum, and this steel is considered to have a superior resistance to chemical corrosion. The stainless steel for run 3 contained a maximum of 0.08% carbon, 17-49% chromium, and 9.l2% nickel. The following data were ob- From these data it is apparent that the phosphoric acid completely inhibited the corrosive action of the sodium chlorite on the ferrous metals.

EXAMPLE 2 C llulose acetate was le che in an aq lur y in several runs wherein various concentrations .of sodium chlorite were employed. 1he pH of each of the slurries was adjusted to 4 with phosphoric acid (about 4.0 parts of phosphoric acid per million parts of slurry). The bleaching process was carried out at a temperature .or' 9095 C. in a stainless steel vessel. The stainless steel was similar to that employed in run 2 of Example 1. After the bleaching operanon the color of the cellulose acetate resulting from each run was determined by mixing 50 parts of bleached cellulose acetate with 15 parts of .d iethyl phthalate and molding the resulting mixture into a test, piece for 15 minutes at 200 C. The color of the resulting test piece was determined in CSCS units. The method for this color determination is based on the method devised by the International Commission on lllumination in 1931 and a complete exposition of the method can be found in the Handbook of Colorirnetry, The Technology Press, Massachusetts Institute of Technology, Cambridge, direction of Arthur C. Hardy. In accordance with this method of color determination the color of unbleached cellulose acetate employed in this example and the subsequent examples ,had a color of 15-20 CSCS units. In the course of the various runs the following data were observed and in this table and subsequent tables p. p. m. indicates parts of sodium chlorite per million parts of slurry.

Table 2 Cellulose Sodium Chlorlte Concentration Acetate Color 350 p. p. m 8.0 700 p. p. m... 8. 9 1,400 p. p. 7. 6

During the course of the various runs no corrosion of the stainless steel was noted.

EXAMPLE 3 Cellulose acetate was bleached in the manner set forth in Example 2, but acetic acid was employed to adjust the pH of the aqueous slurry instead of phosphoric acid. The following data were observed:

Table 3 Cellulose Sodium Chlorite Concentration Acetate Color 180 p. p. m. 10. 1 350 p. p. m 9. 7

At the lower concentration of the sodium chlorite it was noted that the corrosion of the stainless steel vessel was rather slight, but at the higher concentration of the sodium chlorite a severe corrosion of the vessel was observed.

EXAMPLE 4 Cellulose acetate was bleached in the manner set forth in Example 2 but nitric acid was employed to adjust the pH of the slurry instead of phosphoric acid. In a second run no nitric acid was employed for pH adjustment. The color of the cellulose acetate resulting from each of the runs was then noted. The data observed in each of the runs are tabulated below.

The above examples demonstrate the desirability of employing phosphoric acid for pH adjustment in a sodium chlorite bleaching operation of a cellulose ester instead of acetic acid. The above examples also demonstrate that when phosphoric acid is employed in the bleaching process the bleached cellulose ester has a more desirable color than those cellulose esters resulting from operations wherein other acids are employed for pH adjustment. The austenitic alloys employed in the foregoing examples were chosen because of their wide application in chemical vessel and apparatus fabrication.

The corrosion-inhibiting compounds that are employed in the practice of this invention are selected from the group consisting of the phosphoric acids and derivatives of said acids whose aqueous solutions have an acidic pH. Phosphoric acid (H3PO4) is preferred because of its availability, but any of the phosphoric acids can be used. Derivatives of the phosphoric acids within the scope of, this invention are inorganic salts and organic esters of the acids. For example, hydrogen atoms in the acids may be displaced by an ammonium ion or by a metal such as, sodium, potassium, calcium, magnesium, and the like. Also, hydrogen atoms in the acids may be displaced by an organic radical, such as ethyl, propyl, butyl, isobutyl, lauryl, phenyl, tolyl, naphthyl, cresyl, and the like. The derivatives discussed above can be termed acid salts,

ester salts, or acid-ester salts. In any event the derivative of the phosphoric acid retains at least one hydrogen atom, and an aqueous solution of the derivative has an acidic pH.

The amount of corrosion-inhibiting agent employed is variable, depending largely upon the desired pH of the bleaching solution and upon the derivative. Actually, the amount employed is suflicient to adjust the pH of the bleaching solution to the desired range.

In the above specific examples which are illustrative of this invention, sodium chlorite was used as the bleaching agent. Sodium chlorite is the preferred chlorite bleaching agent and it is available commercially. However, the invention is applicable to processes employing chlorine dioxide and other chlorites as bleaching agents. Typical examples or" other chlorites are ammonium chlorite and the alkali metal and the alkaline earth metal chlorites, such as potassium chlorite, calcium chlorite, magnesium chlorite, barium chlorite, and the like.

The bleaching process of this invention is quite useful for bleaching such cellulose derivatives as cellulose esters and cellulose ethers. Typical cellulose derivatives are methyl cellulose, ethyl cellulose, cellulose acetate, cellulose propionate, cellulose butyrate, and the like. Also, mixed ester, mixed ether, and ether-ester derivatives can be bleached in this manner. For example, cellulose acetate propionate, cellulose acetate butyrate, carboxymethyl hydroxyethyl cellulose, acetylated hydroxyethyl cellulose, and the like, are Within the scope of this invention. In its preferred form the invention is applied to the bleaching of cellulose esters, preferably cellulose acetate, with sodium chlorite, and the invention will be described in further detail with reference to such a process.

The cellulose esters employed in practicing this invention can be prepared by any of the known methods. A typical method for preparing these cellulose esters will be set forth by describing a method for preparing cellulose acetate. One method of preparing cellulose acetate involves the preparation of a cellulose mix by spraying cotton linters with glacial acetic acid. An acetylation mix is separately prepared by mixing together acetic anhydride, methylene chloride and catalytic amounts of zinc chloride and perchloric acid. This latter mix and the cellulose mix are combined and agitated for a period of time sufficient to effect the desired acetylation. At the end of this period of time the reaction is arrested by the addition of aqueous acetic acid, and the catalyst is then neutralized with aqueous sodium acetate. The methylene chloride is then removed by distillation and aqueous acetic acid is added until a state of incipient precipitation is reached. Precipitation is then completed by the addition of water and the product is purified by water washing and it is then air dried. Another method of preparing cellulose acetate involves a procedure similar to that described above, but in place of the zinc chloride-perchloric acid catalyst a sulfuric acid catalyst is used. Either of these methods or any other method known for producing cellulose esters is within the scope of this invention.

In practicing this invention the cellulose ester in granular or flake form is suspended in water to form a slurry having a concentration of 7 to 15%. The pH of the slurry is adjusted to within the range of 3 to 6, preferably 4 to 5, by addition to the slurry of an appropriate amount of phosphoric acid. The resulting mixture is heated to a temperature Within the range of 75 to C. and a pressure suificient to maintain the water in the liquid phase is employed. The bleaching metal chlorite is then added to the mixture in the form of an aqueous solution, and the bleaching is allowed to proceed for a period of time sufficient to effect the desired bleaching. Ordinarily, the desired bleaching can be effected in a period of 0.5 to 6 hours, preferably from 1 to 3 hours. During this period of time the mixture is agitated to insure adequate contacting of the cellulose ester and the bleaching metal chlorite, and at the end of this period of time the cellulose ester is drained and washed thoroughly with water.

The bleaching process is effected at the highest slurry concentration that permits efficient agitation and mixing and this varies with the type of agitation available for use. In laboratory procedures the slurry concentrations have generally been below 10% and in commercial practice the usual range of slurry concentrations is from 13 to 15%. By slurry concentration is meant the relation ship between the amount of cellulose ester and the total amount of cellulose ester and slurrying medium.

The amount of bleaching agent that is employed is variable. The prefered amount is the minimum amount required to provide adequate purification of the cellulose ester. The actual amount of bleaching agent employed is within the range of 0.001 to 0.015 part of bleaching agent per part of cellulose ester. \Vood pulp esters require more bleaching than esters of cotton linters. When bleaching Wood pulp cellulose esters, the minimum amount of bleaching agent would not efiect as good a purification as would be effected on a cotton linters cellulose ester. However, with either type of ester, a bleaching thereof would take place With the minimum amount of bleaching agent. The upper limit or 0.015 part of bleaching agent per part of cellulose ester is suflicient to provide an eflicient bleach for cellulose esters of all types in a period of two hours and at a temperature of 95 to 100 C. Greater amounts of bleaching agent can be used, if desired, since the metal chlorites within the scope of this invention are for all practical purposes incapable of oxidatively damaging the cellulose ester. However, greater amounts of bleaching agent are not necessary for carrying out the process.

In the preferred embodiment the bleaching agent is added to the water slurry of the cellulose ester in an aqueous solution. This solution usually has a concentration of bleaching agent within the range of 3 to 5%. However, if desired, higher concentrations of the aqueous solution of the bleaching agent can be used, and it is possible to effect a desired bleaching by adding the bleaching agent to the water slurry of the cellulose ester in the form of a solid. When the bleaching procedure is employed in a process for producing the cellulose ester, such as that described above for preparing cellulose acetate, it is preferred to effect the desired bleaching by contacting the bleaching agent with the cellulose ester at any point in the process after the precipitation of the cellulose ester by addition of excess water. In actual commercial practice the cellulose ester is given a preliminary washing after precipitation with water, and it is then bleached in a water slurry as already described. However, if desired, the cellulose ester can be prepared as a solid in the manner described above, and at a time subsequent to such preparation an aqueous slurry of the cellulose ester can then be prepared. The cellulose ester in this slurry is then bleached in accordance with this invention. When this bleaching procedure is incorporated in the above process for producing a cellulose ester, the cellulose ester, after precipitation with water, is treated by washing with water. A water slurry of the washed ester is then formed, and the pH of the resulting slurry is adjusted to within the desired range with phosphoric acid. The ester is then bleached with sodium chlorite as already described. In an alternative embodiment, an aqueous solution of the phosphoric acid and sodium chlorite in the required amounts can be formed, and the resulting solution is then added to an aqueous slurry of the washed ester to effect the desired bleaching.

From the above disclosure various modifications within the scope of the invention will be apparent to those skilled in the art.

What we claim and desire to protect by Letters Patent 15:

1. The method of improving the thermal color stability of a cellulose derivative selected from the group consisting of cellulose esters and cellulose ethers and of inhibiting the corrosive action of the bleaching agent employed in the process which comprises treating said cellulose derivative with from 0.001 to 0.015 part of a metal chlorite bleaching agent per part of cellulose derivative, said metal chlorite bleaching agent being selected from the group consisting of alkali metal and alkaline earth metal chlorites in contact with stainless steel corrodible by said bleaching agent in an aqueous slurry containing essentially water, said cellulose derivative, said bleaching agent and a phosphoric acid, the pH of said slurry being from 3 to 6, and said bleaching agent and said phosphoric acid being present in amounts suflicient to improve the color of said cellulose derivative and to inhibit the corrosion of said stainless steel.

2. The method of improving the thermal color stability of a cellulose ester and of inhibiting the corrosive action of the alkali metal chlorite bleaching agent employed in the process which comprises treating said cellulose ester with from 0.001 to 0.015 part of alkali metal chlorite per part of cellulose ester in contact with stainess steel corrodible by said alkali metal chlorite in an aqueous slurry containing essentially water, said cellulose ester, said alkali metal chlorite and phosphoric acid, the pH of said slurry being from 3 to 6, and said alkali metal chlorite and said phosphoric acid being present in amounts sufficient to improve the color of said cellulose ester and to inhibit the corrosion of said stainless steel.

3. The method according to claim 2 wherein the cellulose ester is cellulose acetate.

4. The method according to claim 2 wherein the bleaching agent is sodium chlorite.

5. The method according to claim 2 wherein the resulting mixture is agitated for a period of 0.5 to 6 hours at a temperature within the range of to C.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,992,045 Schweitzer Feb. 19, 1935 2,129,719 Vincent Sept. 13, 1938 2,194,956 Taylor Mar. 26, 1940 2,430,674 Hampel Nov. 11, 1947 2,526,839 Aston Oct. 24, 1950 OTHER REFERENCES 25Iaylor et 211., Technical Assn. Papers, 1940, pp. 251- 

1. THE METHOD OF IMPROVING THE THERMAL COLOR STABILITY OF A CELLULOSE DERIVATIVE SELECTED FROM THE GROUP CONSISTING OF CELLULOSE ESTER AND CELLULOSE ETHERS AND OF INHIBITING THE CORROSIVE ACTION OF THE BLEACHING AGENT EMPOLYED IN THE PROCESS WHICH COMPRISES TREATING SAID CELLULOSE DERIVATIVE WITH FROM 0.001 TO 0.015 PART OF A METAL CHLORITE BLEACHING AGENT PER PART OF CELLULOSE DERIVATIVE, SAID METAL CHLORITE BLEACHING AGENT BEING SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL AND ALKALINE EARTH METAL CHLORITES IN CONTACT WITH STAINLESS STEEL CORRODIBLE BY SAID BLEACHING AGENT IN AN AQUEOUS SLURRY CONTAINING ESSENTIALLY WATER, SAID CELLULOSE DERIVATIVE, SAID BLEACHING AGENT AND A PHOSPHORIC ACID, THE PH OF SAID SLURRY BEING FROM 3 TO 6, AND SAND BLEACHING AGENT AND SAID PHOSPHORIC ACID BEING PRESENT IN AMOUNTS SUFFICIENT TO IMPROVE THE COLOR OF SAID CELLULOSE DERIVATIVE AND TO INHIBIT THE CORROSION OF SAID STAINLESS STEEL. 